Many circuits selectively receive inputs from and/or provide outputs to two or more other circuits. A switching circuit that includes transistors may be used to select between the inputs and/or outputs. For example in FIGS. 1A and 1B, first and second circuits 10 and 12 are selectively connected by a switching circuit 14 to a third circuit 16. In some implementations, the first and second circuits 10 and 12 are selectively connected by transistors Q1 and Q2 and Q3 and Q4, respectively. Switching inputs S1 and S1 are used to select the first circuit 10 or the second circuit 12. When S1 is in a first state, the first circuit 10 is connected and the second circuit 12 is not connected. When S1 is in a second state, the second circuit 12 is connected and the first circuit 10 is not connected.
In some situations, the output signal of the first and second circuits 10 and/or 12 may exceed the voltage supply and/or breakdown voltage of the transistors Q1, Q2, Q3 and Q4 that are used in the switching circuit 14. For example, a voltage supply that supplies the switching circuit 14 may provide 2.5V. The switching circuit 14 may be used to switch between first and second transmitters in an Ethernet network device. The voltage output of an exemplary transmitter in a 100BASET network may be operated with a maximum voltage of 3.5V, a minimum voltage of 1.5V, and a common mode voltage of 2.5V. The maximum voltage level of the transmitter outputs may cause operational problems such as breakdown of the transistors Q1, Q2, Q3, and Q4.
Another situation that may require analog switching includes switching between MDI and MDIX configurations in 100BASET or 10BASET network devices. Referring now to FIGS. 2A and 2B, first and second network devices 20 and 22 include physical layers (PHYs) 24 and 26, respectively, that are connected by network cables. For example, the network device 20 can be a personal computer or printer and the network device 22 can be a network switch. Each of the network devices 20 and 22 is connected by at least two pairs of twisted pair wires that are labeled 1, 2 and 3, 6 in FIGS. 2A and 2B.
When in an MDI configuration in FIG. 2A, the PHY 24 has a first pair 1, 2 that is configured as a transmitter 30 and a second pair 3, 6 that is configured as a receiver 34. When in an MDIX configuration in FIG. 2B, the PHY 24 has first pair 1, 2 that is configured as a receiver 46 and a second pair 3, 6 that is configured as a transmitter 48. When in an MDIX configuration, the PHY 26 has a first pair 1, 2 that is configured as a receiver 40 and a second pair 3, 6 that is configured as a transmitter 44. When the network devices 20 and 22 have different configurations, a standard or straight network cable 50 is used. When the network devices 20 and 22 have the same configuration, a crossover network cable 52 is used. When the incorrect network cable is employed for a particular situation (as in FIG. 2B), either the cable must be changed or the transmitter and receiver connections for one of the network devices needs to be switched.